The present invention relates generally to filler rods for fiber optic cables, and more specifically to non-round filler rods containing an increased amount of superabsorbent water swellable material for large-diameter fiber optic cables.
Fiber optic cables have been used by the telecommunications industry for a number of years to transmit information at very high rates over long distances. Fiber optic cables come in a variety of configurations, including: cables with a centrally positioned single buffer tube containing one or more optical fibers; cables with a plurality of buffer tubes stranded in a helical or alternating helical arrangement about a central strength member; and cables with slotted cores in which a plurality of optical fibers reside.
The cables in metropolitan areas are expected to accommodate large numbers of fibers per cable. In particular, the fiber optic ribbon cables containing 864 and 1728 fibers are installed in 1.5 inch diameter ducts. As shown in FIGS. 1 and 2, one way to form such a cable is to use ribbons 2 in relatively large buffer tubes 3. Using a lower number of larger buffer tubes 3 is an efficient way to reduce the overall manufacturing cost of the cable. However, using a lower number of larger buffer tubes 3 produces relatively large gaps between the buffer tubes 3 and the interior surface of the cable outer jacket 1. Also, larger gaps are formed between the buffer tubes 3 and central strength member 8. These large gaps create a problem by allowing for water penetration along the length of the cable.
To overcome this problem by plugging the gaps, which act as water channels, filler rods 4 have been placed between the outer surface of the buffer tubes 3 and the interior surface of the cable outer jacket 1. These filler rods 4 have water swellable material surrounding an inner core. The inner core 5 of the filler rods 4 are generally made of plastic and have a round cross-sectional shape. While use of such filler rods 4 help close the interstitial gaps and to prevent water penetration along the cable, the amount of water swellable material now used is insufficient to plug the gaps or to properly protect the optical fibers, especially in the case of larger diameter cables with larger interstitial gaps. Additionally, the use of cores having round cross-sectional shapes minimizes the surface area to which the water swellable material is adhered. Therefore, it is desired to have filler rods and filler tubes which maximize the amount of water swellable material capable of being adhered thereto.